Process for production of ethylene polymers

Abstract

The invention relates to a process for the production of ethylene polymers by the polymerization of ethylene or the copolymerization of ethylene with one or more C 3 to C 8 olefins in contact with a chromium-based catalyst deposited on a support essentially consisting of silica. According to the invention, the chromium catalyst is subjected in succession to a heat treatment in nitrogen and then to a heat treatment in air under defined temperature conditions before bringing it into contact with ethylene and, optionally, one or more olefins under polymerizing conditions. The process of the invention makes it possible to manufacture ethylene polymers having an improved compromise between processability, stiffness and resistance to slow crack growth.

What is claimed is:  
     
       1. Process for the production of an ethylene polymer or copolymer comprising contacting ethylene alone or in combination with one or more C 3  to C 8  olefins, under polymerization conditions, with a chromium-based catalyst deposited on a support consisting essentially of silica which is substantially free of other oxides, wherein the chromium-based catalyst is subjected to a heat treatment consisting essentially of, in succession, 
       heating in nitrogen at a temperature greater than 350° C. and less than 850° C.,  
       then heating in air at a temperature greater than 350° C. and less than 850° C., and  
       cooling down to room temperature while replacing the air with nitrogen, before the chromium-based catalyst is brought into contact with said ethylene.  
     
     
       2. Process for the production of ethylene polymers according to  claim 1 , wherein the heat treatments, in nitrogen and in air respectively, are carried out a temperature which is at least 480° C. and does not exceed 760° C. 
     
     
       3. Process for the production of ethylene polymers according to  claim 2 , wherein the heat treatments, in nitrogen and in air respectively, are carried out a temperature which is at least 530° C. and does not exceed 715° C. 
     
     
       4. Process for the production of ethylene polymers according to  claim 1 , wherein a duration of the heat treatment in nitrogen is between 10 minutes and 20 hours. 
     
     
       5. Process for the production of ethylene polymers according to  claim 1 , wherein the duration of the heat treatment in air is between 5 minutes and 6 hours. 
     
     
       6. Process for the production of ethylene polymers according to  claim 1 , wherein the support essentially consisting of silica has a BET specific surface area ranging from 100 to 800 m 2 /g and a pore volume ranging from 0.5 to 4 cm 3 /g. 
     
     
       7. Process for the production of ethylene polymers according to  claim 1 , wherein the chromium-based catalyst deposited on a support consisting essentially of silica contains 0.1 to 5% by weight of chromium. 
     
     
       8. Process for the production of ethylene polymers according to  claim 1 , wherein it is applied to the production of ethylene homopolymers or of random or block copolymers of ethylene and butene and/or hexene, containing at least 90 mol % ethylene. 
     
     
       9. Process for the production of ethylene polymers according to  claim 8 , wherein it is applied to the polymerization of ethylene in suspension in isobutane.